This invention relates to a process for treating waste water which contains organic and inorganic substances. In particular the invention relates to treatment of waste water from the production of epichlorohydrin, in which the epichlorohydrin is obtained by reacting dichloropropanol with at least one alkaline compound, preferably an aqueous solution or suspension containing calcium hydroxide, and is separated from the reaction mixture by distillation. In such a process, a waste water remains or can be withdrawn from the synthesis reactor as a bottom product. This waste water contains additional organic, chloro-organic and inorganic compounds as byproducts of the synthesis in addition to very small quantities of the reaction product and the starting material dichloropropanol. This bottom product, which occurs as waste water, typically contains (if calcium hydroxide is used as the alkaline agent in the synthesis of epichlorohydrin) the following compounds: chlorinated, aliphatic, cyclic or alicyclic saturated and/or unsaturated hydrocarbons, saturated and/or unsaturated aliphatic and/or alicyclic chloroethers, chloroalcohols, chloroketones, chloroaldehydes and/or chlorocarboxylic acids, and also, in addition to other compounds which contribute to the COD (chemical oxygen demand) of the waste water, in particular glycerol and glycerol derivatives, and also carboxylic acids, also calcium chloride, calcium carbonate and calcium hydroxide optionally used in excess. The chloro-organic compounds contained in the bottom product contribute to the total AOX (adsorbable organic halogen compound) parameter of the waste water. The AOX is defined as that portion of organic halogen compounds (X=F, Cl, Br, I) which can be adsorbed on activated carbon, the total quantity adsorbed being converted to X=Cl.
The presence of such halogenated organic compounds is a particular problem in the purification of waste water, since the removal of these substances is technically very costly and hence often uneconomic due to the high stability of the covalent carbon-halogen bonds, in particular in the case of sp.sup.2 -bonded halogens. Known measures for reducing halogenated organic compounds in waste water include chemical-physical and biotechnological processes.
Processes for chemical-physical removal of halo-organic compounds from waste water are used for the main purification or preliminary purification (with subsequent biochemical treatment) of the waste water. Processes which are available for this include, for example, activated carbon purification and special extraction processes. The disadvantage of these processes is that they produce a secondary product (loaded activated carbon or extraction agent) laden with halogenated organic compounds. Activated carbon which is loaded with organic contents from waste water from epichlorohydrin production cannot be adequately regenerated by the known method of treatment with steam or hot inert gases, such as nitrogen.
The decomposition of halogenated organic compounds in the biochemical purification stage of a sewage treatment plant likewise raises various problems. In the first place, many of these compounds are poorly susceptible or resistant to biological decomposition by microorganisms, and on the other hand the concentrations of AOX-producing compounds used in the waste water must not be high, and should have substantially constant values.
Additionally, the volume of the activated sludge in such plants is great, and the enrichment of the organic halogen compounds in the sludge is a further problem, so that chemical-thermal processes are often used to destroy halo-organic compounds in waste waters. These include the so-called wet-oxidative methods, in which halogenated organic compounds are decomposed in an oxidizing atmosphere at high temperatures and considerable pressures. However, this method is expensive.
It is known that the extreme physical conditions of chemical-thermal processes can be moderated by the use of catalytically active compounds, in which case these substances may either pass into the system to be dehalogenated by addition of appropriate reagents, or form as intermediate products during the decomposition reaction.
Examples of substances having high reactivity to organically-bonded halogens which are used include metals, metal hydrides or metal alcoholates alone or in combination with a strong base. The disadvantage of the known chemical-thermal processes is, in addition to their relatively high costs, the frequently long reaction times (often more than 10 hours) and the often only moderate decomposition rates.
Furthermore, methods for treating waste waters from pulp bleaching are known in which the chlorolignin compounds contained in the waste water are partially dehalogenated and/or dehydrohalogenated while maintaining particular temperatures, pH values and residence times (See DE-OS 3 620 980 and U.S. Pat. No. 5,120,448). The methods proposed for treating waste water from pulp bleaching cannot be applied to the treatment of waste water from epichlorohydrin synthesis because the two types of waste water have completely different compositions, so that the process conditions, such as pH value, temperature, pressure and residence time, used to treat waste water from pulp bleaching are not transferrable to the treatment of waste water from the production of epichlorohydrin.